Bone marrow transplantation (BMT) represents the best chance for cure for a large number of malignant and non-malignant hematologic diseases. However, BMT implementation is currently limited by the many critical complications that accompany this potentially life-saving therapy. This is especially true for the majority of patients (70-80%) who lack MHC-matched sibling donors, and thus face risky, unrelated and/or MHC-mismatched 'alternative-donor' transplants (AD-BMT). Three major complications have plagued the implementation of AD-BMT. They are: (1) The increased risk of graft rejection. (2) The high rate of acute graft-versus-host-disease (aGvHD) that occurs in the setting of MHC-mismatched BMT; and (3) the profound immunosuppression that patients face after transplant, which renders them highly susceptible to infectious and malignant death in the immediate and long-term post-transplant periods. Prevention and treatment of these three complications represents the major unmet clinical need in BMT. One of the most significant barriers to progress in addressing these complications has been the lack of a preclinical model through which novel biologic therapeutic strategies, developed for human use, can be thoroughly and mechanistically investigated prior to clinical trials. Thus, due to the fact that (1) most novel therapies for BMT (including T cell costimulation blockade, T cell adhesion blockade and adoptive immunotherapy with regulatory T cells) cannot be accurately tested in either murine or canine models for aGvHD and (2) there has been no other translational BMT model available to test these biologics prior to clinical use, they have remained under-analyzed and under-utilized in clinical BMT, despite their burgeoning use in other disease states. In order to address the unmet need for their detailed, translational and mechanistic investigation we have developed a novel primate model of AD-BMT, capable both of dissecting mechanism as well as providing the critical translational bridge to clinical application of novel therapies. In this proposal, we describe experiments using the primate model which will determine both mechanism and efficacy of T cell costimulation blockade, T cell adhesion blockade, and regulatory T cell adoptive immunotherapy on the outcome of AD-BMT.